Crystal Radio Circuits

RF Radio circuits | By: Diagram mania

Crystal Radio Circuits

Simple Crystal RadioOne-Transistor
Amplifier/DetectorTL431 Crystal Radio AmplifierCrystal Radio RF AmplifierVery High Gain Crystal Earphone
AmplifierSimple Two-transistor RadioExperiments with Detector Diodes

Make sure to see the simple Reflex
Receivers! A little more work yields vastly superior results.

Simple Crystal Radio

The crystal radio gets its
name from the galena crystal (lead sulfide) used to rectify the signals. A "cat’s
whisker" wire contact was moved about the surface of the crystal until a diode
junction was formed. The 1N34A germanium diode is the modern substitute for galena and
most other germanium small-signal diodes will also work well. Silicon diodes are not a
good choice because their much higher barrier potential requires larger signals for
efficient rectification. Certain silicon Schottky diodes with low barrier potential will
work well but most small-signal Schottky diodes will not perform as well as a
garden-variety germanium diode.

The circuit is quite simple but many pitfalls await the
novice. The first precaution is most important! The crystal radio works best with a long,
high outdoor antenna but the beginner may not fully appreciate the danger of bringing such
a wire into the house. Lightning strikes to the antenna will probably destroy the crystal
radio but if precautions are not taken, much more damage will result. The best strategy is
to incorporate a commercial lightning arrestor with a straight, heavy gauge ground wire
leading down to a buried water pipe. It is not sufficient to disconnect the antenna from
the receiver during thunderstorms.

Other pitfalls are less dangerous and relate to the
receiver’s performance. A common mistake when building a crystal radio is to load the
tuned circuit excessively. The Q of the tuned circuit must remain high to give selectivity
or strong radio stations will all mix together. A good design will usually have
low-impedance taps on the inductor for connections to the antenna and diode as shown in
the schematic. A long wire antenna with a good ground connection will connect to the
lowest impedance tap whereas a shorter antenna with no ground connection may connect to a
higher tap. The diode may be experimentally moved to different taps and even across the
whole coil for maximum sensitivity. The antenna and diode connection may be made with
alligator clips for easy experimentation.

Another potential problem area is the earphone. Not all
crystal earphones are sensitive and the experimenter should test a few to get a
"good" one. High impedance dynamic earphones are a bit more reliable and can
give excellent results. Try an old telephone receiver or a modern portable tape player
headset (some are high-Z and fairly sensitive). Low impedance earphones like those used
with many portable radios will not work at all. A simple test is to hold one earphone wire
between the fingers while scraping the other lead across a large metal object like a file
cabinet. If static is heard in the earphone it will probably work well with the crystal
radio.

The variable capacitor is often connected incorrectly.
Make sure to connect the rotor to ground and the stator to the "hot" side of the
coil. Otherwise, the radio will detune when the capacitor knob is touched. If detuning is
noticed then try reversing the connections.

Some experimenters are tempted to omit the 82k resistor
which discharges the capacitor on the theory that it wastes precious signal power. With a
typical germanium diode, this little "improvement" may work somewhat but only
because the diode has significant leakage and the performance will not be predictable. A
dynamic earphone may be DC coupled eliminating the need for the resistor.

The coil may be wound on a 1.5 inch PVC pipe coupler as
shown in the drawing. These typically have an outer diameter of about 2.2". Drill two small holes at each end to secure the ends of the coil.
The wire type is not particularly critical but select a gauge and insulation so that the
65 turns cover about 2/3 of the coupler. An excellent choice is 30 AWG "wrap"
wire from Radio Shack. The prototype uses this solid conductor wire with blue
insulation. This wirewrap wire is available in 50′ lengths on little spools and
about 37′ will be needed. A "loopstick" coil may be used in place
of the coil shown. These coils have an adjustable ferrite core for tuning so a fixed value
capacitor may be used in place of the variable capacitor shown. The coil, capacitor and a
terminal strip for the other parts may be mounted to a small wooden board. (See photo of
receiver with transistor amplifier below.)

If a metal chassis is used then the coil must be mounted
horizontally and above the metal to prevent unacceptable loading.

Here are some alternative construction ideas:

Fahnestock clips make excellent connectors for the
antenna and ground wires. The coil may be mounted above the board or chassis
with angle brackets by adding another bend, as shown below. The windings may be
quickly secured with a single layer of colored "Duck" tape that is now available
in more attractive colors than gray or black. The taps to the coil can be
located at the rear, near the bottom so that the unavoidable bulges in the tape
don’t show. An ordinary piece of wood may be quickly finished by applying
adhesive-backed PVC film intended for kitchen cabinets. Just stick it on and
trim flush with scissors.

 

One Transistor Amplifier/Detector

An amplifier may be added to
boost the audio level as shown below. The current consumption of this amplifier is quite
low and a power switch is not included. Disconnect the battery when the receiver is stored
for long periods.

© 1995, Charles Wenzel

Note: You may use the transistor above
as a sensitive detector eliminating the need for the 1N34A diode. Simply leave out the
diode, the 0.001 uF, and the 82k resistor. Connect the negative side of the 1 uF directly
to the coil. Change the base resistor from 10 meg. to 1 meg. and change the collector
resistor from 100k to 10k. Now add a 0.01 uF from the collector to the emitter and the
modifications are complete. This detector is quite sensitive and will be overloaded by
very long antennas! Use a shorter antenna or a coil tap very near ground if significant
distortion is noticed. The circuit draws about 1/2 mA.

Crystal Radio Audio Amplifier

Here is a simple audio amplifier using a
TL431 shunt regulator. The amplifier will provide room-filling volume from an ordinary
crystal radio outfitted with a long-wire antenna and good ground. The circuitry is similar
in complexity to a simple one-transistor radio but the performance is far superior. The
TL431 is available in a TO-92 package and it looks like an ordinary transistor so your
hobbyist friends will be impressed by the volume you are getting with only one transistor!
The amplifier may be used for other projects, too. Higher impedance headphones and
speakers may also be used. An earphone from an old telephone will give ear-splitting
volume and great sensitivity! The 68 ohm resistor may be increased to several hundred ohms
when using high impedance earphones to save battery power.

To use the circuit as a general-purpose
amplifier, apply the input signal to the top of the potentiometer. (Leave out the diode
and .002 uF capacitor.) A higher value potentiometer may be used for a higher input
impedance.

© 1995, Charles Wenzel

Crystal Radio RF Amplifier

For the more experienced hobbyist…

One of the best places to add a transistor
to a simple crystal radio is at the front end in the form of an RF amplifier. The circuit
below is a simple but effective amplifier which will give surprising performance
improvement. This amplifier can exhibit negative resistance for low settings of the 500
ohm pot which results in extra gain or even oscillation. So, the circuit can actually be
considered to be a regenerative receiver with an external detector. The sensitivity is so
high that no cold water pipe ground is needed and the antenna is short.

alt="schematic" width="709" height="326">

The behavior of the amplifier depends on how it is
connected to the tuned circuit. When connected to a lower impedance tap as shown in the
schematic, the gain will be lower with less tendency to oscillate. Higher taps or even
connection directly to the antenna will give higher gain and even oscillation. The 500 ohm
pot is adjusted to give adequate gain without squealing as stations are tuned. High
regeneration settings will actually narrow the bandwidth of the tank enough to give the
sound a "mellow" quality which sounds pretty good in a "tinny" crystal
earphone! Lower settings are best when using an audio amplifier and the fidelity is quite
good thanks to the linear detector (typical regens use changes in the operating point of
the transistor to demodulate the RF). As with any regen, the gain may be increased after
the station is tuned in and the circuit will oscillate, locked to the station’s frequency.

Current consumption is about 1 mA which may be reduced by
increasing the 1.8k but the RF envelope begins to distort below about 500 uA.
A 4.5 volt battery may be used if the 220 k resistor is
reduced to 68 k. The transistor may be just about any NPN small-signal
transistor. No ground is shown but performance is better with a good
ground connected to the bottom of the tuner. Longer antennas should be connected to taps instead of
across the whole coil. A ferrite loopstick will pick up stronger stations with no antenna
at all but use more audio gain after the diode detector and reduce the regeneration to get
adequate bandwidth or the sound will be muffled.

Very High Gain Crystal Earphone Amplifier

This simple, one-transistor amplifier provides a voltage gain over 1000
(60 dB) for driving a high impedance ceramic (crystal) earphone. The high gain is achieved
by replacing the traditional collector resistor with an unusual constant-current diode
that supplies 1/2 mA yet exhibits a very high resistance to the audio. This amplifier will
give excellent battery life, drawing only 500 uA.

schematic

Below is a typical application using it with the first crystal radio
circuit on this page. The amplifier provides good volume with a modest antenna. You may
want a volume control as with the TL431 project!

schematic

Or use the Crystal Radio RF Amplifier directly above for even more
sensitivity with less than 2 mA current drain.

Simple Two-Transistor Radio

Here is a simple radio that was designed to minimize unusual parts; there
isn’t even a detector diode! The sensitivity isn’t as high as the one-transistor reflex
but the simplicity is attractive. Strong stations will provide plenty of volume into a
crystal earphone or an external amplifier.  The AM Loopstick was purchased on eBay
but the enterprising experimenter can swipe one from the interior of a cheap radio.  
If the loopstick has more than one winding, use the one with the most turns. Wind 3 or 4
turns near one end of the winding as seen in the photo. The tuning capacitor in the
prototype is from an old radio and the little plastic dial was cut down such that it just
fit into the back of a black pointer knob. The fit was tight so no glue was needed. All of
the sections of the capacitor were connected in parallel to get the most capacitance for
this loopstick.

All the other parts are common. The transistors can be just about any
small-signal type.  The prototype uses the metal can 2N2222, primarily for looks.
Some transistors may have too much high frequency gain; if the circuit squeals, try adding
a small resistor in the emitter of the first transistor, maybe 47 ohms, the smaller the
better as long as the circuit is stable. The large 47 uF could be smaller in most cases
but the circuit can pick up hum if the wires are too long. Don’t leave out the large
capacitor across the battery, it provides needed low power supply impedance.

schematic

The circuit is built on a piece of 3.8" x 2.7" x 0.5"
stained and varnished oak. The terminals are copper-plated nails used for
weatherstripping. These nails are commonly available in home improvement stores and are
also available in brass which is also solderable. Predrill the holes to make nailing
easier and use a nail set or larger nail turned upside-down to make it easier to hit only
the desired nail. The loopstick is held in position by an ordinary nylon cable clamp and
the battery is mounted in a spring holder.  The front panel is aluminum that was
polished to a nice shine. First sand off all scratches with fine sandpaper. Then remove
the sanding marks with ordinary kitchen steel wool. Now polish the surface with the finest
steel wool in the paint department, usually "000". Then, for the real shine,
polish the surface with a polishing compound like rouge. By the way, those paper-wrapped
sticks of polishing compound are easily dissolved by lighter fluid (naphtha). Just put a
few drops on a tissue and rub it on the end of the stick to load the tissue with compound.
These polishing steps go quickly and you can have a mirror finish in a couple of minutes.

wpeC.jpg (144460 bytes)

The front panel has a couple of scales for the tuning and volume printed
on high gloss report cover stock and sprayed with a protective clear spray. The feet on
the bottom are recessed in the oak using a forstner bit. The corners on the aluminum and
oak were rounded on a belt sander.